The invention relates generally to thin film devices having a functional metal oxide layer, for example, a perovskite ferroelectric memory or high dielectric constant capacitor material. In particular, the invention relates to forming such thin film devices on silicon substrates.
Significant developments have been accomplished in the past few years in the growth and processing of ferroelectric and high dielectric metal oxide thin films for a variety of microelectronic applications. Much of the work has focus on potential integration of these metal oxide films into volatile and non-volatile memories. These efforts blossomed in the early 1980s primarily through pioneering efforts in the sol-gel processing, chemical vapor deposition (CVD), and sputter deposition of multi-component metal oxide thin films. These techniques facilitated the fabrication of sub-micron thin films of ferroelectric materials such as lead zirconate titanate (PZT) and other cationically substituted derivatives of PZT on silicon substrates.
The prototypical structure for a high-density memory cell that evolved from these efforts is schematically illustrated in FIG. 1. A silicon substrate 10 is formed with a large number of memory cells, one of which is illustrated. A source 12 and drain 14 are doped into substrate 10 by ion implantation. A pass gate transistor structure including a gate oxide 16 and metallization 18 is formed over the gate region between the source 12 and drain 14 to form a MOS transistor. Electrical power is supplied to the source 12 by an unillustrated line. The transistor structure is then covered with a first-level dielectric layer 20 typically of SiO2 or a related silicate glass. A contact hole is etched in the oxide dielectric layer 20 over the transistor drain. Polycrystalline silicon is filled into the contact hole to form a silicon plug 22 making electric contact with the transistor drain 14.
The ferroelectric device, in this case, a ferroelectric memory capacitor, is formed over the polysilicon plug 22. The dramatic difference in chemistries between the ferroelectric oxides and silicon necessitates the introduction of a diffusion barrier to eliminate any diffusion of oxygen from the metal oxide ferroelectric layer or other oxide layers to the components of the semiconductor transistor. Even the oxidation of the top of the silicon plug 22 would create a insulative electrical barrier of SiO2 between the ferroelectric capacitor cell and the silicon transistor. The fact that the barrier must be a good electrical conductor and form an ohmic contact to silicon further complicates the selection of barrier materials.
For reasons to be discussed immediately below, a typical barrier consists of a layer 24 of titanium nitride (TiN) and a layer 26 of platinum immediately underlying a lower electrode layer 28. These layers 24, 26, 28 are patterned to form a lower ferroelectric stack. A shaped diffusion barrier layer 30, for example, of titanium oxide (TiO2) is deposited an patterned to have an aperture over the top of the lower ferroelectric stack. A ferroelectric layer 32 is then deposited, for example of PZT or its generalization PLNZT, followed by an upper electrode layer 34, and an upper platinum barrier layer 36. The TiO2 diffusion barrier layer 30, the ferroelectric layer 32, the upper electrode layer 34, and the upper platinum barrier layer 34 are patterned to have larger area than that of the aperture over the lower ferroelectric stack. These depositions complete the ferroelectric stack.
A SiO2 inter-level dielectric layer 38 is deposited and patterned to have a via hole overlying the upper platinum electrode layer 36 of the ferroelectric stack. A contact barrier layer 40, for example of conductive TiN or TiW, is coated at the bottom of the via hole, and a metallization 42, for example, of aluminum or tungsten, is filled into the remainder of the via hole, thereby providing an upper electrical contact to the ferroelectric stack.
Platinum is chosen for the barrier, particularly the lower barrier, primarily because of its refractory nature and resistance to oxidation, unlike, for example, the more common aluminum. Platinum barriers enable ferroelectric capacitors with very desirable basic properties, such as large values of remanent polarization xcex94P, ferroelectric film resistivities of greater than 1010 xcexa9-cm, and sufficient retention characteristics.
Titanium nitride is another obvious choice for a barrier layer, especially since it is already widely used in the semiconductor industry as a diffusion barrier. Unfortunately, TiN oxidizes at about 500xc2x0 C., which is much lower than the optimum process temperature for ferroelectric materials. To overcome the shortcoming of the TiN in terms of temperature, platinum or iridium (Ir) have been used as protective layers. Another common approach is to dope TiN with Al to form (Ti, Al)N or to use suicides or other complex structures. The most common approaches being currently explored use a combination of at least two layers to create a composite barrier layer, such as that in FIG. 1. Taking the PZT ferroelectric material as an example, one approach uses the combination of (Ti, Al)N/(Pt, Ir) as the composite barrier. The structure of FIG. 1 uses a special case of this composite barrier.
However, the above structure presents continuing problems. Even though platinum is a refractory metal and does not oxidize, it is nonetheless fairly porous to oxygen. That is, it does not prevent oxygen from diffusing to the underlying silicon plug and oxidizing a resistive surface layer there. Furthermore, such devices have been observed to suffer fundamental reliability problems. For example, if the test capacitors are repeatedly cycled for more than 107 to 108 bipolar cycles, the amount of remanent polarization still available becomes progressively smaller, and eventually the non-volatile capacitor functionally fails.
The use of platinum or iridium in the barrier or other parts of the stack presents other technological and strategic problems. First, dry etching of Pt or Ir is still very difficult although there have been some recent breakthroughs. A dry etch process, such as reactive ion plasma etching, is considered to be essential for commercial memories to be manufacturable with high yield. Since both Pt and Ir are relatively inert (although Ir does form stable oxides), the ability to form volatile reaction species during dry etching appears to be severely limited. Secondly, both Pt and Ir are considered to be precious metals, not only expensive but of uncertain supply in such quantities required for widespread commercialization. As a result, the economics of supply and demand may impact the feasibility and dependability of using these precious metals in large quantities.
In view of the problems with platinum and iridium, I and others have developed the use of other alloys and compounds that eliminate the need for including these precious metals. The results have been scientifically interesting and offer much promise. In U.S. Pat. No. 5,777,356, Dhote and I describe the use of intermetallic alloys as the conducting barrier layer, without the use of Pt or Ir. An intermetallic alloy is typically composed of two refractory metals in relative compositions that are stoichiometric or nearly so. This approach has been shown to be effective with a Ti/Al-based intermetallic alloy directly contacting the polysilicon plug. A metal oxide, lanthanum strontium cobalate (LSCO) is used as the lower electrode since it has been observed to provide some crystallographic templating for the overgrown PZT ferroelectric. However, the templating is effective only for the out-of-plane orientation, and the in-plane orientation is polycrystalline in a pattern which may be characterized as a mosaic crystal or (001)-texturing
I have suggested another approach in U.S. Pat. No. 5,270,298 in which a barrier layer of ytrria-stabilized zirconia (YSZ) is overlaid by a strongly templating layer of an anisotropic perosvksite such as bismuth titanate before the LSCO electrode is deposited. The templating layer controls the crystallographic orientation and assures the phase stability of the over grown cubic LSCO perovskite layer. This approach has proven very effective, but it requires the deposition of two different layers, namely the buffer and templating layers.
This problem, termed fatigue, fundamentally limits the operation of non-volatile ferroelectric memories since these capacitors are generally used in a mode requiring traversing the hysteresis loop during every write and read operation, hence their designation of destructive read-out memories (DROs).
The problem of fatigue has in large part been overcome through one of two approaches, either using conductive metal oxide electrodes or replacing the PZT with another ferroelectric material, strontium bismuth titanate (SBT). However, the templating provided by conductive metal oxides such as LSCO is incomplete, and the resulting PZT is only (001)-textured. This can be overcome by use of additional layers, such as YSZ or BST, but the number of required deposition steps is increased, and the monocrystallinity of the PZT is still not guaranteed. On the other hand, SBT is disadvantageous.
I and others have recently disclosed a different approach for a related structure in U.S. patent application Ser. No. 09/624,527, filed Jul. 24, 2000, and now issued as U.S. Pat. No. 6,432,546 and incorporated herein by reference in its entirety. In this process, a layer of (Ba, Sr)TiO3 is grown on a crystalline silicon wafer. The preferred composition is strontium titanate (SrTiO3 or STO), which is grown under conditions that the strontium titanate is grown to be epitaxial to the silicon, that is, monocrystalline, and the subsequently deposited LSCO and PZT are also monocrystalline. FIG. 2 shows a hysteresis curve 50 for polycrystalline PZT, hysteresis curve 52 for (001)-textured PZT, and hysteresis curve 54 for epitaxial and monocrystalline PZT grown on strontium titanate that is epitaxially grown on monocrystalline silicon according to the method of the aforecited patent application to Ramesh et al. Clearly, the epitaxial PZT shows the best behavior with both the saturation and remanent polarization increasing with crystallinity. X-ray diffraction data verify the crystalline states of the samples, as described above.
However, strontium titanate cannot be simply added to the memory cell structure of FIG. 1 since strontium titanate is a dielectric or at best a semiconductor, having a room temperature resistivity of somewhat more than 1 ohm-cm, which for a 100 nm or even 1 xcexcm square conduction path amounts to a small strontium titanate capacitor in series with a large PZT capacitor so that substantially the entire voltage drop is across the parasitic STO capacitor. As a result, the use of an STO barrier requires providing another current path into the lower capacitor electrode than through the underlying silicon. The memory cell structure illustrated in the aforecited patent application to Ramesh et al. includes a separate top contact to the bottom LSCO electrode providing a conductive path that avoids the STO layer.
It would be greatly desired to provide a barrier layer over silicon that is epitaxial to silicon and is also electrical conductive.
It would be also desired to extend the concepts of templating STO layers to other classes of devices and to improve upon the known types of STO templating.
According to one aspect of the invention, a conductive barrier layer of (Sr, Ba)(Ti, V, Nb, Ta)O3 is formed between a silicon underlayer and a functional metal oxide layer, such as a ferroelectric device. Preferably, the silicon underlayer is monocrystalline and the functional metal oxide layer and any intermediate metal oxide electrode layers are epitaxial to the silicon underlayer.
According to another aspect of the invention, a barrier layer of (Sr, Ba) (Ti, V, Nb, Ta)O3, which is not necessarily conductive and may include SrTiO3 is used as a templating barrier for the growth of monocrystalline functional metal oxide films used for both non-volatile ferroelectric memories and for other applications.
According a third aspect of the invention, the composition of the (Sr, Ba) (Ti, V, Nb, Ta)O3 film is chosen to be lattice matched to silicon.
According to a fourth aspect of the invention, an epitaxial metallic layer is grown between the silicon underlayer and the conductive barrier layer and may include either an intermetallic alloy or a silicide.